Lithium-doping turns quantum material into conductor and insulator

27 August 2018

This graphic illustrates the effect of inserting lithium ions into the crystal structure of a quantum material called samarium nickelate. Image: Purdue University image/Yifei Sun.

Researchers have studied the effects of shuttling lithium ions back and forth into the crystal structure of the quantum material samarium nickelate. Their findings, which are reported in a paper in the Proceedings of the National Academy of Sciences, suggest a new avenue for research and potential applications in batteries, ‘smart windows’ and brain-inspired computers containing artificial synapses.

Quantum materials are so-called because their performance taps into quantum mechanical interactions. Samarium nickelate is in a class of quantum materials called strongly correlated electron systems, which have exotic electronic and magnetic properties.

The researchers ‘doped’ samarium nickelate with lithium ions, adding the ions to the material's crystal structure. The addition of lithium ions caused the crystal to expand and increase its conduction of the ions. The researchers also learned that the same thing happens with other types of ions, particularly sodium ions, pointing to potential applications in energy storage.

"The results highlight the potential of quantum materials and emergent physics in the design of ion conductors," said Shriram Ramanathan, a professor of materials engineering at Purdue University who is leading the research. "There is a lot of research now going on to identify solid-state ion conductors for building batteries, for example. We showed that this general family of materials can hold these ions, so we established some general principles for the design of these sorts of solid-state ion conductors. We showed that ions like lithium and sodium can move through this solid material, and this opens up new directions for research."

Applying a voltage caused the ions to occupy spaces between atoms in the crystal lattice of the material, which could represent a more efficient method to store and conduct electricity. Such an effect could lead to new types of batteries and artificial synapses in ‘neuromorphic’, or brain-inspired, computers. Moreover, the ions remained in place after the current was turned off, a ‘non-volatile’ behavior that might be harnessed for computer memory.

Adding lithium ions to the crystal structure also changes the optical properties of samarium nickelate. This suggests potential applications as coatings for ‘smart windows’ whose light transmission properties are altered when a voltage is applied.

The paper's lead authors are Purdue materials engineering postdoctoral research associate Yifei Sun and Michele Kotiuga, a postdoctoral fellow in the Department of Physics and Astronomy at Rutgers University. To develop the doping process, they collaborated with Vilas Pol, a Purdue associate professor of chemical engineering and materials engineering, and Purdue graduate student Dawgen Lim.

The researchers were also able to demonstrate behavior related to the ‘Mott transition’, a quantum mechanical effect describing how the addition of electrons can change the conducting properties of a material. "As we add more electrons to the system the material becomes less and less conducting, which makes it a very interesting system to study, and this effect can only be explained through quantum mechanics," Ramanathan said.

Kotiuga's contribution to the work was to study the electronic properties of lithium-doped samarium nickelate, as well as the changes to the crystal structure after doping. "My calculations show that undoped samarium nickelate is a narrow-gapped semiconductor, meaning that even though it is not metallic, electrons can be excited into a conducting state without too much trouble," she said. "As lithium is added to samarium nickelate, the lithium ion will bind to an oxygen and an electron localizes on a nearby nickel-oxygen octahedron, and when an electron has localized on every nickel-oxygen octahedron the material is converted into an insulator. This is a rather counterintuitive result: the added electrons to the system make the material more insulating."

The material's crystal structure was characterized using a synchrotron-radiation light source research facility at Argonne National Laboratory. The researchers now plan to explore further the material's quantum behavior and potential applications in brain-inspired computing.

This story is adapted from material from Purdue University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

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